Peter Phillips and his fiancée, Autumn Kelly, arrive last July at Port Ellen on Islay, Scotland, to embark on a cruise to celebrate the Queen's 80th birthday. ((Andrew Milligan/Associated Press))

The Queen's eldest grandson, Peter Phillips, must give up his right of succession to the English throne if he hopes to marry Montrealer Autumn Kelly.

A British succession law, written in 1701, forbids heirs to the throne from marrying Roman Catholics.Kelly was baptized Catholic at St. John Fisher Parish Church in Pointe Claire, in 1978.

Phillips, the 29-year-old son of the Queen's daughter Princess Anne, will likely renounce his position as 10th in line to the throne in order to marry Kelly. He holds no royal title and does not carry out official royal duties. He works for the Royal Bank of Scotland.

Buckingham Palace announcedthe engagement on July 28. Kelly, a 29-year-old management consultant, met Phillips at the Canadian Grand Prix in Montreal in 2003.

Robert Finch with the Monarchist League of Canada, an organization which supports and promotes awareness of the constitutional monarchy, told CBC News that while the law does bar successors to the throne from marrying Catholics, he didn't see Phillips's situation as a problem.

"I don't think anybody will be losing sleep if Peter Phillips decides to renounce his particular claim to the throne, because the chances of him ever sitting on the throne are so remote," he said.

The royal engagement has stirred up controversy in the United Kingdom. The Archbishop of Scotland and a British Labour MP both have claimed the law is anti-Catholic and have demanded it be changed.

Finch said he thinks the law will change eventually, but only if someone more directly in line to the throne becomes engaged to a Catholic.

Phillips and Kelly have not yet set a date for their wedding.